1. Field of the Invention
The present invention relates generally to initial acquisition of a signal in a communication system which uses a time division multiple access (TDMA) method for transmission. More particularly, the present invention relates to a method and an apparatus for detecting a frequency correction burst, estimating the frequency error and the burst arrival time for a TDMA communication system, such as the Global System for Mobile communications (GSM).
2. Description of Related Art
The TDMA method allows multiple access by subdividing a duration Tf, called the frame duration, into N non-overlapping time slots, each of duration Tf/N. Each user who wishes to transmit information is assigned to a particular time slot within each frame. The Global System for Mobile Communications (GSM) is an exemplary communication system which uses the TDMA method for multiple access.
The Global System for Mobile Communications (GSM), originally known as Groupe Spxc3xa9ciale Mobile, was developed as a standard for cellular communication in Europe. GSM offers a wide range of functionality, transmission of voice and data. It has been adopted throughout Europe and the world. The original primary GSM has been expanded to encompass other versions for wider total bandwidths, such as extended GSM, DCS 1800 and PCs 1900.
GSM is a purely digital system. The primary GSM system, also known as GSM 900, uses the 900 MHz band, of which 890-915 MHz is for mobile transmissions, and 935-960 MHz is for base transmissions. There are 124 channels (174 channels for extended GSM, 374 channels for DCS 1800 or PCS 1900 ) and each channel is 200 kHz wide. The TDMA method is used with 8 time slots, numbered 0 to 7, per channel. GSM uses wide channels to allow high speed digital transmissions, resulting in reducing the effect of fading and minimizing production costs. Gaussian Minimum Shift Keying (GMSK) is used as the modulation process.
The first stage of synchronization between a receiver and a transmitter is called the initial acquisition process. In a mobile communication environment, there are always initial time and frequency errors due to uncompensated local oscillator (LO) error, range uncertainty between the transmitter and the receiver, and motion in a multi-path channel environment. Upon completion of initial acquisition, the time and frequency errors are reduced to within acceptable limits.
In GSM, a base station uses a Broadcast Common Control Channel (BCCH) to transmit signaling information. As shown in FIG. 1, the frame structure of the control channel includes 51 TDMA frames lasting 235.38 milliseconds (each TDMA frame is 4.615 ms in duration). Within this repetitive frame structure, each of the information data shown in FIG. 1 is transmitted in time slot 0 of a corresponding TDMA frame. Synchronization data include a Frequency correction Burst (FB) and a Synchronization Burst (SB). Within 235.38 ms, there are 5 FBs, appearing in time slots 0 of frames 0, 10, 20, 30,40.
The FB is a sine wave at 67.7 kHz above the carrier, lasting 546.12 xcexcs. The FB is formed by differentially encoding a string of 148 bits of values xe2x80x9c0xe2x80x9d and then modulating the resulting 147 bits. The data structure of the string of 148 bits is as shown in FIG. 2. The string consists of 142 bits (all 0s) preceded by 3 tailing bits (three 0s) and followed by three tailing bits (three 0s) and 8.25 guarding bits (all 1s).
In the acquisition mode, the receiver has to detect the FBs and perform timing and frequency error estimation. The receiver then uses the frequency error estimate to correct the reference frequency of the local oscillator, and the timing estimate in the subsequent demodulation of the Synchronization Burst (SB) which is sent in time slots 0 of frames 1, 11, 21, 31, 41.
In the acquisition mode, a receiver first scans the whole frequency band used for GSM (or extended GSM, or DCS 1800 or PCS 1900 ) and measures the relative signal strength on up to 124 channels (174 channels for extended GSM, 374 channels for DCS 1800 or PCS 1900 ), each channel having a bandwidth of 200 kHz, to obtain a list of the strongest channels. Then, the receiver will start to search for the FBs over the channels in the list.
According to the GSM standard, the required frequency accuracy is 0.1 ppm, which is 90 Hz for a 900 MHz carrier. Assuming that the frequency stability of the crystal oscillator of a receiver is 5 parts per million (ppm) (including aging and temperature factors, etc.), the maximum initial frequency offset can be about 5 kHz for GSM 900, and about 10 kHz for DCS 1800 and PCS 1900, when the receiver is first turned on. This initial frequency error must be rapidly detected and reduced to within the GSM requirement.
Accordingly, there is a need for a method and an apparatus to detect the FBs and estimate the initial frequency error in real time with high accuracy in order to meet the GSM standard.
The present invention is a method and: an apparatus for detecting a frequency correction burst embedded in a signal which includes ordered samples. The method comprises the following: (a) correlating K sequences of the ordered samples with a predetermined waveform to produce K correlation outputs, each of the K correlation outputs corresponding to one of the K sequences, having a magnitude and being associated with an order index, K being an integer, the predetermined waveform corresponding to the frequency correction burst; (b) storing the K correlation outputs in a buffer according to the order indices; and (c) estimating a parameter based on the K correlation outputs, the parameter indicating detection of the frequency correction burst.
The estimated parameter includes a frequency error and an ending time of a frequency correction burst. A quality metric associated with each frequency error is also computed.